The purpose of adding mineral reinforcements to polymers in order to obtain different composites has been mainly to fulfill a functional requirement such as increasing stiffness or reducing manufacturing costs.
The reinforcements used in polypropylene composites have usually been talc and calcium carbonate of mineral origin, and to a smaller extent mica and fibers. Usually, the addition of mineral reinforcements has an effect on the fragility of the polymer composite and decreases the impact energy. Therefore, the addition of rigid particles to polypropylene leads to a loss of flexibility in the polymer.
Alternatives for the development of polypropylene composites having better stiffness, impact resistance and crystallinity can be obtained by chemically modifying the polymer (which implies changing the molecular structure by the incorporation of functional groups or molecules in the polymerization process or by an insertion reaction additional to polymerization) as well as by the type of reinforcement used.
Polypropylene composites may be obtained by the use of resistant reinforcements such as fibers and/or by the incorporation of reinforcements of submicrometer particle size to reduce the plastic resistance of the composite.
In the manufacture process of a polypropylene composite, the molecular weight range or the melt flow index of polypropylene must be considered, as well as the homogeneous distribution of the reinforcement in the polymer matrix, in such a way that the maximum compatibility is achieved between the hydrophilic reinforcement and the hydrophobic matrix. Maximum interfacial adhesion must also be achieved between the reinforcement and the polymer matrix to optimize the mechanical and thermal properties of the composite. Usually this compatibility between the reinforcement and the polymer matrix can be improved by using polypropylenes that have different melt flow indices as well as by using reinforcements that have submicrometer particle size with a narrow particle size distribution.
The most commonly used reinforcements in polypropylene composites are calcium carbonate or talc, both having mineral origin, and it is possible to obtain polypropylene composites by mixing in an extrusion molding process. Polypropylene composites with these mineral reinforcements have better mechanical behavior than polypropylene alone. When a mineral material or chemical compound such as calcium carbonate or talc is incorporated, the cost decreases because of the lower use of polypropylene in the composite. With this invention, however, the production cost of polypropylene composites has been optimized, since a waste material, eggshells, is used as reinforcement.
Egg processing plants around the world use billions of eggs every year, depositing thousands of tons of eggshells in dump sites. In Chile 2,500 million eggs are produced per year, of which about 10% are meant for industrial use. This implies that annual industrial eggshell waste is in the order of 1,500 tons.
No information has been found in the state of the art on polypropylene composites with eggshell reinforcement, nor on the use of eggshells to obtain a natural reinforcement as shown in this invention.
The manufacture of polypropylene composites using eggshell reinforcement developed in this invention has the following advantages when compared to polypropylene composites using traditional and commercial natural reinforcements like calcium carbonate or talc:                The composites of this invention combine the properties of the polymer matrix and the advantages of using an agroindustrial waste such as eggshells to obtain a natural type of reinforcement that has availability, recyclability, efficient use of energy, environmental benefits, and low cost.        The composite of the invention has a higher Young's modulus, which means greater stiffness, than the polypropylene composite using a calcium carbonate reinforcement with the smallest commercially available average particle size (d(50)=0.50 μm).        The composites of this invention have lower density, greater crystallinity and better mechanical behavior to traction and impact than polypropylene composites using calcium carbonate.        The natural reinforcement makes it possible to replace up to 75% by weight of total talc with the natural reinforcement in polypropylene composites, yet retaining the high stiffness of composites using submicrometer talc with an average particle size d(50)=0.50 μm. Thus, less talc is used as reinforcement in the polypropylene composite and the density of the composite decreases by up to 10% at the same time.        There is high compatibility and good adhesion between the natural reinforcement and polypropylene using polypropylenes that have different melt flow indices.        Lower rate of decrease of the energy absorbed on impact as the proportion of reinforcement in the polypropylene composite increases, compared with traditional reinforcements.        
In addition, the natural reinforcement used to obtain polypropylene composites according to this invention has the following advantages with respect to traditional commercial mineral reinforcements such as calcium carbonate and talc:                A chemical composition whose main component, calcium carbonate, corresponds to one of the traditional mineral reinforcements most widely used in polypropylene composites.        A calcite crystallographic structure like that of mineral calcium carbonate.        Its average particle size is in the submicrometer range and it has a narrow particle size distribution, appropriate for use as reinforcement in a polypropylene matrix.        The morphology of its particles is laminar and therefore it allows better orientation in the polypropylene matrix.        Lower density than the traditional mineral reinforcements such as calcium carbonate and talc used as reinforcements in polypropylene.        Lower cost than the traditional mineral reinforcements used in polypropylene composites.        No technological use that may give it an added value: it is just an agroindustrial waste without further usefulness.        